This invention in general relates to interferometry and in particular to interferometric apparatus and methods by which the altitude between a datum surface and a referent surface may be measured as at least part of the surfaces may be moving relative to one another.
Interferometry is a well established metrology used extensively in microfabrication processes to measure and control a host of critical dimensions. It is especially important in manufacturing semiconductors and the like where requirements for precision are 10 to 40% better than critical dimensions of 0.1 μm or below.
Integrated circuits made of semiconductor materials are constructed by successively depositing and patterning layers of different materials on a silicon wafer while it typically resides in a nominally flat exposure plane having Cartesian x-y coordinates with a normal z-direction. The patterning process consists of combinations of exposure and development of photoresist followed by etching and doping of the underlying layers and then deposition of another layer. This process results in a complex and, on the scale of microns, very nonhomogeneous material structure on the wafer surface.
Typically each wafer contains multiple copies of the same pattern called “fields” arrayed on the wafer in a nominally rectilinear distribution known as the “grid.” Often, but not always, each field corresponds to a single “chip.”
The exposure process consists of projecting the image of the next layer pattern onto (and into) the photoresist that has been spun onto the wafer. For the integrated circuit to function properly each successive projected image must be accurately matched to the patterns already on the wafer. The process of determining the position, orientation, and distortion of the patterns already on the wafer, and then placing them in the correct relation to the projected image, is termed “alignment.” The actual outcome, i.e., how accurately each successive patterned layer is matched to the previous layers, is termed “overlay.”
In general, the alignment process requires both translational and rotational positioning of the wafer and/or the projected image as well as some distortion of the image to match the actual shape of the patterns already present. The fact that the wafer and the image need to be positioned correctly to get one pattern on top of the other is obvious. Actual distortion of the image is often needed as well. Other effects, such as thermal and vibration, may require compensation as well.
The net consequence of all this is that the shape of the first-level pattern printed on the wafer is not ideal and all subsequent patterns must, to the extent possible, be adjusted to fit the overall shape of the first-level printed pattern. Different exposure tools have different capabilities to account for these effects, but, in general, the distortions or shape variations that can be accounted for include x and y magnification and skew. These distortions, when combined with translation and rotation, make up the complete set of linear transformations in the plane.
Since the problem is to successively match the projected image to the patterns already on the wafer, and not simply to position the wafer itself, the exposure tool must effectively be able to detect or infer the relative position, orientation, and distortion of both the wafer patterns themselves and the projected image.
It is difficult to directly sense circuit patterns themselves, and therefore, alignment is accomplished by adding fiducial or “alignment marks” to the circuit patterns. These alignment marks can be used to determine the reticle position, orientation, and distortion and/or the projected image position, orientation, and distortion. They can also be printed on the wafer along with the circuit pattern and hence can be used to determine the wafer pattern position, orientation, and distortion. Alignment marks generally consist of one or more clear or opaque lines on the reticle, which then become “trenches” or “mesas” when printed on the wafer. But more complex structures such as gratings, which are simply periodic arrays of trenches and/or mesas, and checkerboard patterns are also used. Alignment marks are usually located either along the edges of “kerf” of each field or a few “master marks” are distributed across the wafer. Although alignment marks are necessary, they are not part of the chip circuitry and therefore, from the chip maker's point of view, they waste valuable wafer area or “real estate.” This drives alignment marks to be as small as possible, and they are often less than a few hundred micrometers on a side.
Alignment sensors are incorporated into the exposure tool to “see” alignment marks. Generally there are separate sensors for the wafer, the reticle, and/or the projected image itself. Depending on the overall alignment strategy, these sensors may be entirely separate systems or they may be effectively combined into a single sensor. For example, a sensor that can see the projected image directly would nominally be “blind” with respect to wafer marks and hence a separate wafer sensor is required. But a sensor that “looks” at the wafer through the reticle alignment marks themselves is essentially performing reticle and wafer alignment simultaneously and hence no separate reticle sensor is necessary. Note that in this case the positions of the alignment marks in the projected image are being inferred from the positions of the reticle alignment marks and a careful calibration of reticle to image positions must have been performed before the alignment step.
Furthermore, as implied above, essentially all exposure tools use sensors that detect the wafer alignment marks optically. That is, the sensors project light at one or more wavelengths onto the wafer and detect the scattering/diffraction from the alignment marks as a function of position in the wafer plane. Many types of alignment sensor are in common use and their optical configurations cover the full spectrum from simple microscopes to heterodyne grating interferometers. Also, since different sensor configurations operate better or worse on given wafer types, most exposure tools carry more than one sensor configuration to allow for good overlay on the widest possible range of wafer types.
The overall job of an alignment sensor is to determine the position of each of a given subset of all the alignment marks on a wafer in a coordinate system fixed with respect to the exposure tool. These position data are then used in either of two generic ways, termed “global” and “field-by-field,” to perform alignment. In global alignment, the marks in only a few fields are located by the alignment sensor(s) and the data are combined in a best-fit sense to determine the optimum alignment of all the fields on the wafer. In field-by-field alignment the data collected from a single field are used to align only that field. Global alignment is usually both faster, because not all the fields on the wafer are located, and less sensitive to noise, because it combines all the data together to find a best overall fit. But, since the results of the best fit are used in a feed-forward or dead reckoning approach, it does rely on the overall optomechanical stability of the exposure tool.
Alignment is generally implemented as a two-step process; that is, a fine alignment step with an accuracy of tens of nanometers follows an initial coarse alignment step with an accuracy of microns, and alignment requires positioning the wafer in all six degrees of freedom: three translation and three rotation. But adjusting the wafer so that it lies in the projected image plane, i.e., leveling and focusing the wafer, which involves one translational degree of freedom (motion along the optic axis, the z-axis or a parallel normal to the x-y wafer orientation) and two rotational degrees of freedom (orienting the plane of the wafer to be parallel to the projected image plane), is generally considered separate from alignment. Only in-plane translation (two degrees of freedom) and rotation about the projection optic axis (one degree of freedom) are commonly meant when referring to alignment. The reason for this separation in nomenclature is the difference in accuracy required. The accuracy required for in-plane translation and rotation generally needs to be on the order of several tens of nanometers or about 20 to 30% of the minimum feature size or critical dimension (CD) to be printed on the wafer. Current state-of-the-art CD values are on the order of several hundred nanometers and thus the required alignment accuracy is less than 100 nm. On the other hand, the accuracy required for out-of-plane translation and rotation is related to the total usable depth of focus of the exposure tool, which is generally close to the CD value. Thus, out-of-plane focusing and leveling the wafer require less accuracy than in-plane alignment. Also, the sensors for focusing and leveling are usually completely separate from the “alignment sensors” and focusing and leveling do not usually rely on patterns on the wafer. Only the wafer surface or its surrogate needs to be sensed. Nevertheless, this is still a substantial task requiring, among other things, precise knowledge about the vertical position (the altitude) of the optical projection system above the wafer. To achieve this vertical position measurement, interferometers are known as that described, for example, in U.S. Pat. No. 6,020,964. This interferometer, however, appears to suffer from a measurement beam not being parallel with an associated reference beam after only a single measurement beam pass to a measurement object, a significant shear of the interferometer's reference and measurement beams at the detector due to non-parallelism of the measurement and associated reference beams after a single pass to the measurement object for converting information carried on optical signals to electrical form, and from environmental and air turbulence effects in portions of the measurement beam paths not directly associated with the altitude.
Another important source of error in certain interferometers for measuring altitude arises as a result of the presence of optical components that can introduce undesirable phase shifts in measurement and/or reference beams. Such phase shifts, if uncompensated, result in amplitude errors and can contribute to cyclic errors as well.
Accordingly, it is a primary object of the present invention to provide phase compensation features to correct for undesirable phase shifts introduced between beam components by certain kinds of reflections.
It is another object of the present invention to provide phase compensation means to correct for undesirable phase shifts introduced in measurement and/or reference beams in interferometers.
It is another object of the present invention to provide interferometric apparatus and methods by which the altitude of photolithographic optical system above a wafer may be precisely measured with minimal beam shear due to non-parallelism of a measurement and associated reference beams after a single pass to the measurement object and with reduced non-parallelism of the measurement and associated reference beams after a single pass to the measurement object.
It is another object of the present invention to provide interferometric apparatus and methods by which the altitude of photolithographic optical system above a wafer may be precisely measured with minimal beam shear and with reduced non-parallelism of the measurement and associated reference beams after a single pass to the measurement object while being sensitive to changes in the index of refraction due to environmental and turbulence effects of a medium only along the altitude portion of a measurement path and not sensitive to changes in the index of refraction due to environmental and turbulence effects of a medium only along other portions of a measurement.
It is another object of the of the present invention to provide interferometric apparatus and methods by which the altitude of photolithographic optical system above a wafer may be precisely measured by looking down on the wafer from the optical system or up from the wafer to the optical system.
It is another object of the of the present invention to provide interferometric apparatus and methods by which the altitude of photolithographic optical system above a wafer may be precisely measured with x and y translations of the wafer not introducing any Doppler shifts in the frequency of an optical signal carrying altitude information.
Other objects of the present invention will, in part, be obvious and will, in part, appear hereinafter when reading the following detailed description in conjunction with the drawings.